The present invention relates to air-fuel ratio detecting systems, and more particularly the invention relates to a system for detecting the air-fuel ratio of an engine from the composition of its exhaust gases.
Air-fuel ratio detecting systems have heretofore been proposed which, as shown for example in U.S. Application Ser. No. 018,776 entitled "Air-Fuel Ratio Detecting System" filed in the names of Hattori, et al on Mar. 8, 1979, comprises an air-fuel ratio sensor comprising mainly of a metal oxide semiconductor such as titania (TiO.sub.2) which senses the air-fuel ratio A/F from the exhaust gas component, such as, the concentration of oxygen in the exhaust gases of an engine and whose electric resistance value varies in dependence on the oxygen concentration, fixed dividing resistors connected to the air-fuel ratio sensor, and a comparator circuit for comparing the voltage at the junction of the sensor and the resistors with a predetermined reference voltage so as to determine whether the air-fuel ratio A/F is greater or smaller than the stoichiometric air-fuel ratio.
However, this type of known system is disadvantageous in that since the dividing resistors are set to fixed values, in dependence on the operating temperature and aging the electric resistance value Re characteristic of the air-fuel ratio sensor changes (or shifts) on the whole as, for example, from the curve X to the curve Y in FIG. 1, thus deteriorating the detection accuracy of air-fuel ratio A/F and increasing the tendency to cause erroneous detection.